Thermal evaporation is a well-known approach to forming a number of materials such as III-V solid-state semiconductors via molecular beam epitaxial (MBE) growth. Another commercial application of this technique is the evaporation of aluminum (Al) onto polymer foils for the packaging industry or other metals onto polymer foils for capacitor manufacturing. In these applications, the sources are typically point sources either of the Knudsen cell design or the open boat design. Point sources are also used in manufacturing of thin-film photovoltaic (PV) devices, in particular copper indium gallium selenide (CuInxGa1-xSe2 or CIGS) devices. In addition, large area organic light-emitting diode (OLED) devices are often fabricated using thermal evaporation sources. Due to their large-area substrates and required uniformity of the deposited layers, thermal evaporation sources utilized for OLEDs are typically of the linear type.
Some materials commonly deposited via thermal evaporation are difficult to heat up or cool down due to their poor heat capacity and poor thermal conductivity. Prime examples of such materials are phosphorous (P), sulfur (S), and selenium (Se). All of these elements have low heat capacities common for most solid elements coupled with rather poor thermal conductivities. Other group V elements, in particular arsenic (As) and antimony (Sb), also common in thermal evaporation, exhibit two orders of magnitude higher thermal conductivities. Tellurium (Te) is also of importance in various applications and has a moderate thermal conductivity one order of magnitude lower than As and P and one order of magnitude higher than P, S, and Se. Table 1 below provides exemplary values of heat capacities and thermal conductivities for these materials. In comparison, copper (Cu) has a molar heat capacity of 24.44 J mol−1 K−1 (0.385 J g−1 K−1) and a thermal conductivity of 401 W m−1 K−1 at room temperature.
TABLE 1MolarSpecificThermalHeat capacityHeat CapacityConductivityElement[J · mol−1 · K−1][J · g−1 · K−1][W · m−1 · K−1]P23.820.7690.236S22.750.7050.205As24.570.32850Se25.350.3210.519Sb25.200.20724Te25.650.2013
Furthermore, the temperature/vapor pressure relationship for P, S, As, Se, and Te is significant when compared to metals such as Sb, Cu, or refractory metals such as molybdenum (Mo). As FIG. 1 shows, the vapor pressures of P, S, As, Se, and Te increase very rapidly with increasing temperature. In summary, thermal energy couples poorly into P, S, and Se (and Te), but when it does the vapor pressure of the material increases rapidly. These thermal properties for P, S, and Se complicate control of the evaporation rate in a thermal-evaporation process for these elements. In addition, the heat-up and cool-down times for large amounts of feedstock of these materials are long.
Given the cost sensitivity of commercial products—in particular for PV—manufacturing requires long system run times and short system turnaround (“green-to-green”) times. Thus, evaporation sources typically hold significant volumes of feedstock to enable long-run campaigns. Coupled with the desire to increase throughput, high deposition rates and large-area substrates are essential to enabling lower manufacturing costs. Therefore, conventional high-throughput thermal evaporation sources have significant thermal mass and/or utilize continuous feed of the source material. For some materials continuous feed is a possibility (e.g., Al wire feed), while for many others it is not.
Conventional systems with high thermal mass have the added advantage that control of the thermal evaporation process is simplified as temperature fluctuations based on power fluctuations to the heaters are typically negligible. Highly effective thermal insulation further reduces sensitivity to incoming power fluctuations. Such thermal insulation also reduces heat losses to the surroundings, i.e., it increases thermal coupling efficiency of the electrical heater power to the material to be evaporated, leading to lower operating costs. In summary, high thermal mass and highly effective thermal insulation are important aspects of conventional industrial thermal evaporation processes.
In addition, turnaround times typically need to be short for industrial deposition processes. However, if the thermal evaporation source has a high thermal mass and highly effective insulation, the cool-down of the source between deposition runs will necessarily be slow. The impact is most severe if an unscheduled maintenance event necessitates shutdown of the equipment with the large-volume sources still holding significant amounts of feedstock. But even if the feedstock has been depleted, the bodies of the evaporation sources themselves still have significant thermal mass.
In view of the foregoing, there is a need for improved thermal-management systems and techniques for thermal evaporation that maintain high-quality insulation (and concomitant insensitivity to power fluctuations) during deposition cycles and that provide faster cooling and higher turnaround times between deposition cycles.